The present invention relates to fluid controllers, and more particularly, to such controllers which are of the dynamic load signal type, and/or of the open-center type.
It will become apparent to those skilled in the art that the present invention may be used advantageously with various types and configurations of fluid controllers which are operable to control the flow of fluid from a source (such as a pump and load sensing priority valve) to a fluid pressure operated device (such as a vehicle steering cylinder). The invention is especially useful when applied to a fluid controller of the type in which the source of pressurized fluid communicates a dynamic load signal (as opposed to a "static" load signal) to the fluid controller, and the invention will be described in connection therewith. A dynamic load signal system of the type to which the present invention relates is illustrated and described in U.S. Pat. No. 4,620,416, assigned to the assignee of the present invention and incorporated herein by reference. However, the invention may also be used advantageously with an open-center controller, and that embodiment will also be described subsequently.
In a typical fluid controller of the type to which the present invention relates, there is a first flow path from an inlet port to a fluid meter and a second flow path from the meter to a first control port, which is connected to one side of the cylinder. The other side of the cylinder is connected to a second control port which is connected by a third flow path to a return port. The first and second flow paths define first and second variable flow control orifices, respectively, while the third flow path typically defines a third variable flow control orifice, although such is not essential to the present invention. These variable flow control orifices are generally referred to as the A1, the A4, and the A5 orifices, respectively, and will so be referenced sometimes hereinafter. In a typical fluid controller of the type used for full fluid-linked steering of an off-highway vehicle, the second variable orifice (A4) begins to open, and then subsequently, the first variable orifice (A1) begins to open. This phased opening of the control orifices is done partly to prevent the trapping of fluid within the controller.
In a controller of the dynamic signal Type, as disclosed in the above-cited patent, the dynamic signal is typically communicated to the first flow path, downstream of the first variable flow control orifice. The first flow path and the dynamic load signal path are in communication with the return port by means of a load signal drain orifice which, typically, is open when the valving is in its neutral position, and begins to close as the valving is displaced. In the dynamic load signal fluid controllers sold commercially by the assignee of the present invention, the variable load signal drain orifice remains open, and does not fully close, until after the first and second variable flow control orifices have begun to open.
Although dynamic load signal fluid controllers of the type described have been in commercial use and have been successful, an undesirable condition normally referred to as "drift" has been observed under certain operating conditions. It has been observed that, when the controller valving is being displaced only slightly, i.e., to make small steering corrections, the condition of the valving is as follows. The first variable flow control orifice is either closed or open very slightly, the second variable flow control orifice is open, and the load sense drain orifice is still open. In this condition, steering into a load results in reverse flow from the steering cylinder, through the second variable orifice, then through the fluid meter, then through the load sense drain orifice to the system reservoir. Such a reverse flow can result in a loss of registry between the position of the steering wheel anti the position of the steering cylinder and steered wheels, i.e., the steered wheels "drift" relative to the position of the steering wheel. This drift condition is especially likely if the rotary element of the fluid meter is unbalanced axially, such as by hydraulic forces and rubs against an adjacent housing member or end cap, resulting in drag on the rotatable member. When this occurs, it is possible to have leakage through the fluid meter, and past the rotatable member of the meter.
Another manifestation of such reverse flow is a phenomenon known as "wheel kick", in which the reverse flow through the fluid meter imposes a load on the rotatable member of the meter, with the load being transmitted mechanically back to the steering wheel, where it is felt by the vehicle operator as an undesirable, high force movement of the wheel in a direction opposite to the direction in which the operator is turning the wheel.
It has been observed that both drift and wheel kick occur only until the load sense drain orifice closes, thus shutting off the path for fluid to flow in a reverse direction. The drift and wheel kick have also been observed to occur only when the first variable flow control orifice is closed, or open very slightly, such that the fluid controller is not able to build sufficient pressure, downstream of the A1 orifice, to meet or exceed the load pressure. By definition, in a load sensing system, the inlet pressure should always be greater by a predetermined differential than the load pressure, but this is true, as a practical matter, only after the A1 orifice is open a sufficient amount to build pressure downstream thereof.
Similar phenomenon have been observed in open-center controllers in which there is a variable neutral orifice which gradually closes as the A1 orifice gradually opens, thus permitting the possibility of reverse flow through the neutral orifice, under conditions similar to those noted above.